Within this application several publications are referenced by Arabic numerals within parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entirety are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The different functional classes of T lymphocytes recognize antigen on the surface of distinct populations of target cells. Helper T cells interact largely with macrophages and B cells; cytotoxic T cells interact with a broader range of antigen-bearing target cells. These cellular recognition events are likely to be mediated by the specific association of surface molecules on both effector and target cells. The surface of T cells is characterized by a number of polymorphic, as well as nonpolymorphic, proteins which are restricted for the most part to T lymphocytes. Although most of these molecules are common to all T cells, two classes of surface proteins consistently differ on the different functional classes of T cells, and these proteins have been implicated in T cell-target cell interactions.
One class of surface molecules distinguishes the major functional subsets of T lymphocytes: the surface glycoproteins T4 and T8. Early in thymic development, the glycoproteins T4 and T8 are coexpressed on the surface of thymocytes (1). In the peripheral immune system, the T4 and T8 molecules are expressed on mutually exclusive subsets of T cells and are only rarely expressed on the same cell (2, 3). The T4 molecule is expressed on T cells that interact with targets bearing class II major histocompatibility complex (MHC) molecules, whereas T8-bearing T cells interact with targets expressing class I MHC proteins (4, 5, 6, 7, 8, 9). The T4 population of T lymphocytes contains helper cells, whereas the T8 population contains the majority of cytotoxic and suppressor cells (6, 10). However, rare T4.sup.+ T cells can function as cytotoxic or supressor cells (6, 10), suggesting that the expression of T4 or T8 is more stringently associated with MHC class recognition than with effector function. The significance of these molecules in T cell-target cell interactions can be demonstrated by studies with monoclonal antibodies. Antibodies directed against specific epitopes of the T4 molecule (or the murine equivalent L3T4) inhibit antigen-induced T cell proliferation, lymphokine release and helper cell function (7, 8, 11, 12, 13). Similarly, monoclonal antibodies directed against T8 (or the murine equivalent Lyt2) inhibit cytotoxic T cell-mediated killing (14, 15). These observations, along with the fact that T4 and T8 do not reveal significant polymorphism, has led to the hypothesis that T4 and T8 recognize nonpolymorphic regions of class II and class I molecules, respectively.
A second class of proteins thought to differ on different effector T cells are the receptors that recognize antigen in association with polymorphic regions of MHC molecules (16, 17, 18). The interactions of helper T lymphocytes are largely restricted to antigen-bearing target cells expressing class II MHC proteins, whereas cytotoxic and suppressor T cells are restricted to targets bearing class I MHC molecules (4, 5, 6, 7, 8, 9). These specific interactions may be mediated by the T cell receptor (or receptors) that recognize antigen in the context of specific MHC molecules (17, 18). Thus, the T lymphocyte may have two independent receptors capable of recognizing both constant and polymorphic determinants of MHC proteins, and these receptors may be responsible for specific targeting of functionally distinct populations of T cells.
The human acquired immune deficiency syndrome (AIDS) is characterized by a depletion of T4.sup.+ lymphocytes. As a consequence, T cell-mediated immunity is impaired in AIDS patients, resulting in the occurrence of severe opportunistic infections and unusual neoplasms. AIDS results from the infection of T lymphocytes with a collection of closely related retroviruses (LAV, HTLV-III, or ARV), now termed human immunodeficiency virus (HIV). The range of infectivity of these agents is restricted to cells expressing the T4 glycoprotein on their surface.
Therefore, the T4 glycoprotein may serve not only as a receptor for molecules on the surface of target cells, but also as a receptor for the AIDS virus. Monoclonal antibodies directed against T4 block AIDS virus infection of T4.sup.+ cells in vitro. Furthermore, recent studies nave demonstrated that when T4.sup.+ T lymphocytes are exposed to AIDS virus, the 110 kd envelope glycoprotein of the virus is associated with the T4 molecule on the host cell. The lymphotropic character of the virus could therefore be explained by the restricted expression of its receptor, T4, in subpopulations of T lymphocytes.
The depletion of T4.sup.+ T lymphocytes in AIDS results in the impairment of the cellular immune response. In addition, AIDS is frequently accompanied by central nervous system (CNS) dysfunction, most often the consequence of a subacute encephalitis. AIDS virus RNA and DNA has been identified in affected brains, and virus has been isolated from both brain and cerebrospinal fluid from patients with neurological disorders. These observations suggest that the AIDS virus infects brain cells and is directly responsible for the CNS lesions observed in AIDS patients. Thus, the AIDS virus may be neurotropic as well as lymphotropic. It is therefore important to determine whether T4 is also expressed in the CNS or whether additional brain-specific surface molecules may serve as a receptor for the AIDS virus.
The elucidation of the specific interactions of T4 and T8 would be facilitated by the isolation of the T4 and T8 genes, the determination of their structure, and the ability to introduce them into different cellular environments. The isolation and sequence of a cDNA encoding the T8 molecule has recently been reported (19, 20, 21). The deduced protein sequence indicates that T8 is a membrane-bound glycoprotein with an N-terminal domain that bears homology to the variable region of immunoglobulin light chains.